64-bit Travis tests failing non-deterministically after Travis upgrade

Split from the 32-bit crashes of #2634 (closed)

For 64-bit, a completely different subset of tests fails each run. The tests have zero output as #2640 shows. I cannot reproduce in a VM that is similar to the Travis VM.

debug-internal-64: 273 tests passed, **** 2 tests failed: ****
	code_api|linux.clone 
	code_api|linux.exit 

https://travis-ci.org/DynamoRIO/dynamorio/jobs/276032337

debug-internal-64: 273 tests passed, **** 2 tests failed: ****
	code_api|sample.prefetch 
	code_api|sample.stl_test 

https://travis-ci.org/DynamoRIO/dynamorio/jobs/277148078

debug-internal-64: 265 tests passed, **** 10 tests failed, but ignoring 0 for i: ****
	code_api|linux.signal0000 
	code_api|float_vmbase 
	code_api|client.count-ctis 
	code_api|client.syscall 
	code_api|client.count-bbs 
	code_api|client.exception 
	code_api|client.annotation-concurrency-opt 
	code_api|client.annotation-detection 
	code_api|sample.prefetch 
	code_api|tool.reuse